In this thread on the macosxhints forum site, clskang asked about synching a Microsoft Mobile 5 phone via VirtualPC. While this doesn't work, I had success using the recently-released GuestPC 1.8.
I posted my results in the above-linked forum thread, including information on how to get Windows2000 Service Pack 3 installed on GuestPC. This link will jump directly to my post.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060406015858680